Switch from sundown to commonmark

[yihui]

This is to fix https://github.com/yihui/knitr/issues/1329.

Also close #76.

TODO:

• [ ] https://github.com/PolMine/polmineR/pull/232
• [ ] https://github.com/PolMine/polmineR/pull/233
• [x] https://github.com/rstudio/learnr/pull/745
• [x] https://github.com/rstudio/leaflet/pull/823
• [x] https://github.com/rstudio/htmltools/pull/342
• [x] https://github.com/kiernann/gluedown/pull/29
• [x] https://github.com/rstudio/plumbertableau/pull/84
• [x] https://github.com/datacamp/tutorial/pull/31
• [x] https://github.com/AllanCameron/geomtextpath/pull/85

[cderv]

Awesome @yihui!

Thanks for doing it !